The present invention relates to an apparatus for measuring the configuration of a fine flat surface two-dimensionally or three-dimensionally in a wide dynamic range, and more particularly to an apparatus for measuring the precise configuration of a free curved surface such as the surface of an aspherical lens, the configuration of the fine pattern of a semiconductor, and the configuration of a Fresnel lens having fine steps such as a grating formed on the curved surface thereof.
It is impossible for an interferometer or a conventional contact type three-dimensional configuration measuring apparatus to measure the configuration of a free curved surface such as that of an aspherical lens to the accuracy of a sub-micron to 10 nm. In order to overcome this problem, Japanese Laid-Open Patent Publications No. 57-189761 (corresponding to U.S. Pat. No. 4,611,916) and No. 60-148715 (corresponding to U.S. Pat. No. 4,776,699) have disclosed an optical measuring apparatus for measuring the configuration of an aspherical surface and a free curved surface with a high accuracy. These apparatuses comprise an optical probe for measuring the configuration of an object surface by means of lights which have been condensed on the object surface and reflected thereby.
According to the above optical apparatus, the resolving power is limited in the horizontal direction of a spot because lights are condensed on the object surface. Therefore, the configuration of the surface of an area as accurate to not more than 1 .mu.m cannot be measured. In addition, since the configuration of the object surface is measured by means of lights reflected by the object surface, the apparatus is incapable of measuring the configuration of a surface coated with a film of a nonreflective material. This problem depends on a principle of the optical apparatus.
In order for a contact type measuring apparatus to measure the configuration of an object surface with a high accuracy, it is necessary to consider not only the straightness of an object placing stage, and the accuracy and position of a scale, but also the contact pressure between the object surface and a probe, or a pin which contacts the object surface. It is necessary to provide the apparatus with a probe having a radius of curvature not more than 0.1 .mu.m to measure the object with resolving power of as fine as 0.1 .mu.m in a horizontal direction. In order for the probe not to deform the object surface, the contact pressure is required to be not more than approximately 30.about.100 nN (3.about.10 .mu.g) depending on the material of the surface of an object.
The above-described contact pressure range cannot be obtained by a conventional probe due to its unfavorable mass and the unfavorable constant of a spring. That is, if a contact pressure is chosen according only to the weight of the probe, the amount of diamond mounted on the lower end portion of the probe and used as the contact pin is as small as 0.1 mm.sup.3. If a tungsten leaf spring of 13 .mu.m in thickness, 0.25 mm in width, and 3 mm in length is used to support the probe, the constant thereof is 1 N/m. A force of 30 nN is required to be applied to the leaf spring to flex it 30 nm. It is extremely difficult to keep the contact pressure constant because the probe is required to move in conformity with the object surface which is irregular. Because of the above reason, it is very difficult for the probe-provided conventional apparatus to measure the configuration of the object surface under a small contact pressure.
In order to measure the object surface of a precise configuration, there is provided a scanning type tunnel microscope which actively controls the pin position of the probe based on the quantity of tunnel current which flows through the object surface. But the microscope is used to measure a surface composed of limited kinds of metals and the range to be measured thereby at a time is very small, because a piezo element is used to control the pin position.